1. Field of the Invention
This invention relates to reflective erbium-doped amplifiers (R-EDAs) and more specifically to compact reflective EDAs that use ultra-short high-gain waveguides.
2. Description of the Related Art
Significant and on-going efforts are being made on erbium-doped fiber amplifier (EDFA) schemes to improve amplifications characteristics such as gain, noise figure, saturation output power, and so on. One of the amplifier schemes used to achieve high signal gain is reflective-type EDFA (R-EDFA) as described by S. Nishi et al, xe2x80x9cHighly efficient configuration of erbium-doped fiber amplifierxe2x80x9d, ECOC""90, vol. 1 (Amsterdam), 1990, pp. 99-102. As shown in FIG. 1a herein, R-EDFAs 10 with a 3-port optical circulator 12 and a mirror 14 placed at each end of the coiled silica erbium-doped fiber 16, respectively, give double-path amplification to input optical signals. A single-mode pump 18 is coupled to fiber 16 via a WDM coupler 20 to pump the active material in the fiber core. An input optical signal is provided at port 1 22, which directs the signal out of port 2 24 to the EDF. The reflected signal is returned to port 2, which then directs the signal out of port 3 28.
A conventional 3-port circulator 12 of the type described in U.S. Pat. No. 4,650,289 by Kuwahara is illustrated in FIG. 1b herein. This is a schematic depiction of a typical circulator, which can be implemented with many different combinations of optical elements, see for example U.S. Pat. No. 6,178,044. The conventional optical circulator includes four ports, port 1 22, port 2 24, port 3 28, and port 4 30, which is terminated to define a 3-port circulator. The optical circulator also includes polarizer prisms 32 and 34, mirrored prisms 36 and 38, Faraday rotators 40 and 42, optically active elements 44 and 46, and a collimating lens 26. The polarizer prisms 32 and 34 transmit light in different directions depending on the polarization of the light.
The polarization of any optical signal can be divided into two mutually orthogonal directions, both of which are also perpendicular to the direction of propagation of the light. Light polarized in the first direction is transmitted undeflected by the polarizer prisms 32 and 34. Light polarized in the second direction is transmitted at an angle of ninety degrees from the first direction. The mirrored prisms 36 and 38 merely reflect light without a change in polarization. The Faraday rotators 40 and 42 rotate the direction of polarization of incident light by forty-five degrees in a particular direction regardless of the direction in which light traverses the Faraday rotators. For example, the Faraday rotator 40 rotates the polarization of light from the prism 38 in the same direction as light from the optically active element 44. Optically active elements 44 and 46 rotate the polarization of incident light by forty-five degrees. However, the direction that the polarization is rotated depends upon the direction in which the light traverses the optically active elements 44 and 46. For example, optically active element 44 will rotate light from the Faraday rotator 40 and having one polarization by forty-five degrees in a particular direction. The optically active element 44 will rotate light from the polarizer prism 34 having the same polarization by forty-five degrees in the opposite direction Thus, an optical signal incident on the port 1 22 will travel a path through the mirrored prism 36, a path through the optically active element 44, of a path depending on the polarization of the optical signal. However, the elements of the conventional optical circulator 12 are chosen such that the portion of the optical signal from port 1 22 that is reflected from prism 38 will have a polarization such that it will be transmitted at ninety degrees by the polarizer prism 34. Similarly, the elements of the conventional optical circulator 12 are chosen such that the portion of the optical signal from port 1 22 that is transmitted by the optically active element 44 will have a polarization such that it will be transmitted undeflected by the polarizer prism 34. Thus, an optical signal from port 1 22 will reach port 2 24, but not be transmitted to port 3 28 or port 4 30 and similarly for each of the ports except that port 4 is terminated.
R-EDFAs that incorporate optical circulators provide significant gain improvement primarily due to double passage of the signal through the erbium-doped fiber. Reflection of the pump results in higher average inversion ratio. However, only about 20% of the single-mode core-pumped radiation is not absorbed or scattered in the meters of silica fiber on the first pass, and is available for reflection through a second pass. Thus, the effect of reflecting the pump in a silica fiber amplifier is marginal.
Typically, tens of meters of silica fiber is coiled to obtain the desired amplification. The bend radius of the fiber is typically at least 50 mm to avoid attenuation. Integrated optical systems will require compact optical components, hence smaller bend radii. The induced attenuation due to bending a SMF28 single mode fiber is 0.5 dB per turn for a 16 mm bend radius with a single-mode core pumping at 1550 nm. In cladding pumped amplifiers the limitations on bend radius are even more severe since pump light can more readily escape the cladding than the core. In addition, the bending may redistribute the pump mode shape to favor modes with smaller or no overlap with the centrally-doped core, resulting in lower pump absorption and reduced gain for the amplifier.
Furthermore, Nishi""s R-EDFA exhibits severely degraded noise figure compared with the conventional single-pass EDFA because the amplified signal and backward amplified spontaneous emission (ASE) make the population inversion in the input part of the EDF low. J. Ahn et. al. xe2x80x9cTwo-Stage reflective-type erbium-doped fiber amplifier with enhanced noise figure characteristicsxe2x80x9d, Optics Communications 197 (2000) pp. 121-125 Sep. 15, 2001, describes a two-stage R-EDFA to enhance noise figure. An positions the circulator to split the EDF into two segments, which prevents the amplified signal and backward ASE from propagating to the first segment. As a result, the population inversion in the first segment remains high and the noise figure is better than the conventional R-EDFA. A small amount of amplification is sacrificed.
U.S. Pat. No. 5,757,541 to Fidric entitled xe2x80x9cMethod and Apparatus for an Optical Fiber Amplifierxe2x80x9d splits the pump light and input signal into two equal parts for simultaneous introduction into the two opposite ends of the active gain fiber. This bi-directional propagation results in a more uniform excitation along the entire length of the active fiber, providing uniform stimulation of photon emission at both ends, causing significantly reduced noise and higher gain of the signal. Fidric""s OFA is not an R-EDFA and thus does not realize the enhanced gain associated with the signal passing through the active fiber twice.
U.S. Pat. No. 5,598,294 to Uno et al. entitled xe2x80x9cOptical Fiber Amplifier and Optical Fiber Communication Systemxe2x80x9d describes a R-EDFA in which the mirror is replaced by one or more wavelength selective reflectors to gain equalize different wavelength signals. This construct allows ASE outside the signal wavelengths to pass thereby improving noise figure. In one configuration, the last wavelength selective reflector is specified to reflect the pump wavelength.
U.S. Pat. No. 5,596,448 to Onaka et al. entitled xe2x80x9cDispersion Compensator and Optical Amplifierxe2x80x9d provides an optical amplifier, which is not influenced by chromatic dispersion or polarization mode dispersion. As shown in FIG. 8 therein, a dispersion compensation fiber 4 is connected in cascade with the EDF in an R-EDFA configuration. The dispersion compensation fiber has color dispersion of a sign opposite that of the silica telecom fiber, and the length thereof is set so as to conform to the value of the color dispersion of the silica telecom fiber.
The reflection of the optical signal in a conventional high gain ( greater than 20 dB) silica EDFA may cause lasing in the amplifier, which is a fatal condition for an amplifier. The expense of including a circulator more than offsets any savings in fiber cost or a lower power pump. Since the silica fiber is spooled, the reflective architecture does not change the form factor of the amplifier. Finally, the complexity and expense associated with recycling the single-mode core pump is not warranted because only a relatively small amount of the pump energy is available for recycling.
In view of the above problems, the present invention provides a reflective optical amplifier that supports novel multi-mode pumping schemes, compact array configurations and integration with reflective optical components at a low per port cost for mid-gain applications.
This is accomplished by replacing the meters of coiled silica fiber with ultra-short high-gain waveguides formed of co-doped erbium-ytterbium multi-component glass a few centimeters in length. The multi-component glasses support doping concentrations of the rare-earth ions erbium and ytterbium far in excess of levels believed possible with conventional glasses. These dopant levels in combination with the reflective scheme make a compact R-EDA with sufficient amplification possible. The waveguides may be planar waveguides, optic fibers or a hybrid fiber-waveguide array.
Unlike conventional silica R-EDFAs, the integration of the reflective architecture with the ultra-short waveguide does not produce lasing, greatly enhances pump efficiency, and reduces the form factor. The compact amplifier is targeted at mid-gain ( less than 20 dB) applications so lasing is avoided. Greater than 40% and probably about 70% of the multi-mode clad-pumped radiation is not absorbed or scattered in the ultra-short waveguide on the first pass and is available for reflection through a second pass. Since the waveguide is preferably linear and not spooled, the double-pass architecture either reduces the form factor for a given performance or enhances performance for the given form factor.
The compact R-EDA is pumped using non-conventional multi-mode pumps that couple to the waveguide cladding. A multi-mode pump can be coupled via a fused-fiber coupler, Total Internal Reflection (TIR) coupler mounted on the waveguide, to the open end of the waveguide, from the side of the waveguide or through a modified circulator. The waveguide""s ultra-short form factor allows each of these pumping schemes to be configured into an array by placing a mux/demux between the circulator and a waveguide array. The compact R-EDA and R-EDA array are also well suited for integration with other reflection-type optical components such as gain flattening filters, dispersion compensators, variable optical attenuators (VOAs) or monitors.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: